Inverter Replacement on a Feed-in Tariff System

If your solar inverter has failed and your system is on the Feed-in Tariff, the reassuring news is this: replacing the inverter does not end your FIT. Ofgem's rules expressly allow generating equipment to be repaired or replaced, and for solar PV that equipment means the panels and the inverter.

What you do need to get right is the choice of replacement and the paperwork that goes with it. There are three realistic routes, and they have different consequences for your metering and your FIT payments.

Key points

• Replacing a failed inverter does not, in itself, affect your FIT accreditation.

• Your FIT licensee should be told about the replacement, and your network operator (DNO) needs notifying when the new inverter is commissioned.

• A like-for-like replacement is the simplest route for your FIT.

• Upgrading to a hybrid inverter lets you add a battery, but it changes how your system has to be metered, and that can reduce your FIT generation payments.

• If you are paid metered export rather than deemed export, take advice before adding any battery that can charge from the grid.

Why FIT-era inverters are failing now

Most Feed-in Tariff systems were installed between 2010 and 2019. Panels are built to last a long time, but inverters are not. MCS says that while panels can last over 25 years, the inverter may need replacing during that time. The Energy Saving Trust puts a typical inverter life at around 10 to 15 years.

So a system from the early 2010s arriving at an inverter failure is not bad luck. It is roughly what the equipment was expected to do.

What Ofgem says about replacing equipment

Ofgem's guidance for FIT generators says you may repair or replace all or some generating equipment without affecting the compliance of an accredited installation, provided the installation continues to meet the scheme rules. In practice that means it keeps generating from the same technology, at the same site, and is not decommissioned or moved.

Ofgem reinforced this in a 2021 decision: the act of replacing generating equipment does not, in itself, mean the installation has been decommissioned.

Two points worth understanding:

• Your installed capacity is based on your panels. Ofgem calculates the Total Installed Capacity of a solar installation from the rated output of the modules multiplied by the number of modules. Swapping the inverter does not change the panels, so it should not change your installed capacity or your tariff rate.

• Adding panels is different. New panel capacity commissioned on or after 15 January 2016 is not eligible for FIT payments. If you are thinking of adding panels at the same time, read our guide to keeping your FIT when you upgrade first.

Even for a like-for-like swap, tell your FIT licensee. Ofgem lists replacing generating equipment among the changes generators should report.

The DNO side

Your inverter is the part of the system your network operator cares about, because it is what connects to the grid.

Under the current connection standard for small systems, EREC G98, an older system that has a significant piece of equipment such as an inverter replaced has to be brought up to the current standard. In plain terms: the new inverter must be a type-tested, compliant unit, and your installer notifies the DNO within 28 days of commissioning it.

If the new inverter, or the new inverter together with a battery, takes your system above 16 amps per phase (3.68 kW on a normal single-phase supply), a G99 application is needed before installation instead. That is a longer process, so it is worth knowing early.

Your three options

1. Repair

Repair is worth asking about, but it is often not available. Many domestic string inverters are sealed units designed to be replaced rather than repaired, and whether repair is possible depends on the manufacturer still supporting that model with parts and firmware. Some brands common in FIT-era systems have changed hands or left the UK market since.

If a repair is genuinely possible and the inverter is otherwise sound, it can make sense. If the unit is at the end of its expected life, a repair may only postpone the replacement.

2. Like-for-like replacement

A new string inverter of a suitable rating, doing the same job as the old one.

For your FIT this is the cleanest route. Your generation meter carries on measuring only what your panels produce, your installed capacity is unchanged, and nothing about your export arrangement needs to change. The paperwork is a notification to your FIT licensee and a G98 notification to the DNO.

3. Upgrade to a hybrid inverter

A hybrid inverter can run your panels and a battery, either now or later. For many households this is a natural time to make that change, because the old inverter has to come off anyway.

The catch is metering. With a hybrid inverter the battery sits before your generation meter, and if the battery can charge from the grid, your FIT licensee has to be satisfied that the generation meter is not counting grid electricity as generation. Ofgem's guidance on co-located storage says this can be satisfied with a bi-directional meter that measures electricity in and out and displays a net value. On hybrid replacements, that is what we fit.

It is important to understand the trade-off. Because that meter records a net figure, Ofgem notes that battery charging losses are likely to reduce your FIT generation payments somewhat, and in a month where the net figure is negative, no FIT can be claimed for that month. A standard smart meter does not count as a bi-directional meter for this purpose. If the battery is set so that it can only charge from your panels, Ofgem's guidance says the existing generation meter can stay.

And another way to get a battery: keep a working inverter

If your existing inverter is healthy and you simply want storage, a battery can be added alongside it as an AC-coupled system. The battery then sits after your generation meter, so the meter still measures only your panels and your FIT generation readings are unaffected. We cover this in our guide to adding a battery to existing solar panels.

The export payment question

This deserves a plain explanation.

Most FIT-era homes are paid deemed export: a set share of your generation is treated as exported, whatever the real figure. Ofgem's guidance says deemed export can continue with co-located storage, provided the generation meter measures only your panels.

If you are paid metered export from an export meter, it is different. Ofgem's guidance on co-located storage says that where it is not possible to identify whether the export meter is measuring export from the FIT installation or from another source, such as battery electricity imported from the grid, you are not entitled to export payments. Whether any alternative, such as deemed export, is available is for your FIT licensee to decide, so ask before the battery goes in.

Ofgem also notes that if a smart meter is fitted and you were receiving deemed export payments, those stop and you are paid for measured export instead. If you are planning a battery and a smart meter, get the metering right before any work starts.

What to do next

1. Note any fault code on the inverter display, and take a photo.

2. Find your FIT statement so you know your licensee and whether you are on deemed or metered export.

3. Find your original installation paperwork if you have it.

4. Book an inspection. We will confirm the inverter has actually failed, check the rest of the system, and set out the options and what each means for your FIT.

Frequently asked questions

Will replacing my inverter stop my FIT payments?

Not in itself. Ofgem allows equipment to be repaired or replaced provided the installation continues to meet the scheme rules. Tell your FIT licensee about the change.

Does a bigger inverter increase my FIT?

No. Your installed capacity for FIT is based on your panels, not your inverter.

Can I add a battery at the same time?

Yes, and it is often the sensible moment. How it is connected decides what metering you need and how your FIT readings are affected. If you are on metered export, get advice first.

Do I need to tell my DNO?

Yes. Your installer notifies the DNO when the new inverter is commissioned, or applies beforehand if the new setup exceeds 16 amps per phase.

Can you replace an inverter on a system you did not install?

Yes.

Book a solar inspection. If your inverter has stopped, we will find out why and tell you honestly whether to repair, replace or upgrade.